1. Field of the Invention
In one aspect, this invention relates to a method of feeding a two phase vapor-liquid feed mixture to a fractionator. In another aspect, this invention relates to a method of feeding a wide-boiling range mixture to a fractionator. In still another aspect, this invention relates to an improved fractionator feed section.
2. Background
Fractionation is used to separate feed mixtures into various distillate and residual streams. In certain applications, the fractionator feed can be a two-phase vapor-liquid mixture. In other applications, feed to the fractionator can have a wide-boiling range. It is common in petroleum or heavy oil fractionation to have complex feeds and to feed a two-phase mixture to a fractionator or to have a fractionator feed which generates a significant volume of vapor in the fractionator feed section. For example, with crude tower feed, it is generally desirable to preheat the crude in a furnace to the maximum allowable temperature prior to feeding the crude to the crude tower. Since crude oil contains a quantity of relatively light and volatile hydrocarbons as well as many much heavier components, the preheated crude tower feed can be two-phase. Also, as another example of complex feeds, various refinery heavy oil streams can be combined with reactor effluent streams, recycle oil streams and other streams having components with widely differing boiling points to form a fractionator feed. This feed can result in significant feed generated vapor in the fractionator feed section, especially when large concentrations of light hydrocarbons are fed to a relatively high temperature zone of the fractionator.
Generally, fractionators contain a number of trays where ascending vapor in the fractionating tower is contacted with liquid cascading down the tower, which contact causes heat and mass interchange on each tray. Each tray comprises a downcomer to direct liquid from such tray to the tray below, a weir to maintain a liquid level on such tray, perforations to permit vapor passage upward through such tray, and a contacting means such as bubble caps, valve tray, or the like to enhance liquid-vapor contact. The vapor becomes progressively lighter as it rises through the tower and the liquid becomes progressively heavier as it passes down the tower. The vapor leaving the tower is totally or partially condensed and generally at least a portion of the condensed stream is returned back to the top tray of the tower as reflux and a portion is recovered as distillate product.
Fractionating towers can contain one-, two-, three- or four-pass trays. In a single-pass tray configuration, there is only one flow path for liquid across each tray and downward from the tray as the downcomer is located only on one side of the tray. For a two-pass tray configuration, two flow paths are available for the liquid. Accordingly, two types of trays are used in a two-pass fractionating tower, one of which is a center downcomer tray and the second is a side downcomer tray. Three-pass and four-pass configurations are variations of the foregoing.
In many prior art fractionating processes, the fractionator feed tray is selected such that the feed mixture is fed to a zone of the fractionator where the feed temperature is similar to the temperature of the vapor-liquid mixture on the tray. This usually results in the smallest diameter fractionator and also saves energy. Further, the feed can thus be introduced to the column with minimal upset to the column if only a relatively modest portion of the feed is vaporized on the feed tray.
In a typical equipment arrangement for a fractionator feed section, the fractionator is equipped with multiple feed nozzles. Each feed nozzle is directed to a different tray, each of which can be selected as the feed tray. The feed is then fed through the desired nozzle. The multiple nozzle arrangement permits changing of feed location to enable the feed to be directed to a portion of the column where its introduction will not result in operational instability. Typically, in such arrangement, tray spacing is uniform. Generally, the feedstock is introduced to the tray selected as the feed tray on the side of the feed tray side opposite to the feed tray downcomer to obtain reasonable mixing of the feed with the effluent passing through the downcomer from the tray above the feed tray and to provide for some retention or "hold-up" time for liquid-vapor contact on the feed tray. However, in crude and heavy oil fractionation, changing location of the feed to the tower generally does not reduce problems associated with inadequate mixing, entrainment and instability.
Unstable operation and liquid entrainment are problems associated with feeding a feed mixture with a relatively large concentration of volatile components to a high temperature zone of a fractionator. Rapid gaseous expansion and a large quantity vapor flow occur in the feed section of a prior art fractionating column to which a relatively volatile feed mixture is fed. If the feed comprises large concentrations of low boiling components, upsets from vapor expansion occur in the feed section. As large amounts of ascending vapor from the feed tray pass through the liquid on the tray above the feed tray, droplets of liquid will be entrained into such ascending vapor. As the vapor velocity is increased in the feed section of the column due the gaseous expansion of the volatile feed, entrainment and carry over of droplets increases, and column separation efficiency and capacity are limited thereby. Frequently, the spacing between the feed tray and the tray above the feed tray are set at the same tray spacing as all of the trays in the column; however, in certain applications in prior art columns, the distance between the feed tray and the tray above are increased to minimize liquid entrainment. This increase in tray spacing results in a loss of the number of trays available for separation and loss of fractionator capacity and/or efficiency.
It is desirable to have a method of feeding a relatively volatile feed mixture to a fractionator. It is also desirable to have a fractionating column feed section which reduces entrainment of liquid caused by vaporization of feed in the feed section and achieves good mixing of the feed with the liquid and vapor to the feed tray.